Methanol is produced by contacting a synthesis gas containing at least carbon monoxide, carbon dioxide and hydrogen with a catalyst. This synthesis gas is converted to methanol in a separate vessel.
The synthesis gas (syngas) can be produced either by reforming a hydrocarbon with steam or by reacting hydrocarbons with oxygen or by a mixture of these two technologies used in series or in parallel. The syngas produced in the syngas production unit or units is then compressed with a syngas compressor before being converted to methanol in a converter which is operated at a high pressure in the range of 80–120 bar. The unreacted syngas is then compressed in a syngas recycle compressor, mixed with the syngas coming from the syngas production unit and the mixed stream is sent to the inlet of the methanol converter. The methanol is recovered and purified in a methanol recovery unit made essentially of two or three distillation columns in order to reach the required methanol purity.
Since methanol is produced at high pressures in the range of 65–120 bar, the main process steps usually include syngas production, syngas compression in a dual stage synthesis gas compressor, methanol synthesis and methanol recovery. When oxygen is used to produce synthesis gas, there is an air separation unit upstream of the syngas production unit.
The current trend is to produce methanol in very large quantities, five thousand tonnes of methanol per day or more. Large methanol plants require large syngas production units and the trend is to use oxygen-based technologies in order to produce these large quantities of syngas in a single train unit and to increase the energy efficiency of the overall methanol production process. Alternatively, it is possible to produce large quantities of syngas without using oxygen by injecting large quantities of carbon dioxide in the synthesis gas production unit. The carbon dioxide may come from various sources such as the carbon dioxide present at the outlet of the methanol converter or the carbon dioxide made in nearby ammonia production units or the carbon dioxide naturally present in natural gas. With a large CO2 injection, it is possible to reach an energy efficiency comparable to those obtained with oxygen-based schemes.
Most of these processes use an air separation unit (ASU) to produce oxygen at high pressure to convert natural gas to synthesis gas, the synthesis gas being sent to a single converter where it is converted to methanol.
As described in U.S. Pat. No. 6,117,916, an ASU produces oxygen at 40 bar and the oxygen reacts with steam and natural gas in a partial oxidation reactor to produce synthesis gas. The synthesis gas at 40 bar is then compressed to 70 bar and is sent to a methanol reactor, thereby producing methanol at 66 bar.
Usually, the synthesis gas compressors have to include two stages since the pressure gap between the current reactor and the current converter is too large for pressure to be met with a single stage compressor. These dual stages compressors are more expensive, both in capital investment and in terms of upkeep. Furthermore, two synthesis gas compressors in parallel are often used to avoid maintenance problems.